This invention relates to vehicle braking systems and more particularly, to an improved manual-braking mode.
There is a continuing effort to improve the performance of vehicle braking systems in both the powered and unpowered mode. Power or power assisted braking often uses a fluid pressure actuator such as a vacuum booster to boost the driver-exerted force on the brake pedal. Such a known power braking system 18 is schematically illustrated in FIG. 2. Depressing a brake pedal 20 moves a pushrod 22 to the right as illustrated in FIG. 2. The pushrod is connected to a piston rod (not shown) in a master cylinder 24 in a known manner. The pushrod 22 extends through a vacuum booster 26 that contains a diaphragm 28 mechanically connected to the pushrod 22. The diaphragm 28 separates and seals a forward fluid chamber 36 from a rear fluid chamber 38. The forward fluid chamber 36 is connected to the vehicle vacuum system 30, and the rear fluid chamber 38 is vented to atmosphere via a port 32. As the operator pushes the brake pedal 20, the pushrod 22 operates a control valve (not shown) in a known manner to open the forward chamber 36 to the vacuum system 30. Thus, atmospheric pressure in the rearward chamber 38 applies a substantial force on the diaphragm 28, which translates with the pushrod 22. Thus, the vacuum booster 26 provides a substantial additive force to the force supplied by the operator on the brake pedal 20.
Known master cylinders 24 have primary and secondary cylinder portions that are operated simultaneously and in parallel in response to the translation of the pushrod 22. Therefore, the master cylinder 24 provides two independently operable fluid pressure outlets 40, 42. Hydraulic fluid from the first output is fluidly connected to a first pair of the wheel cylinders 44, 46, and hydraulic fluid from the second master cylinder output 42 is fluidly connected to a second pair of the wheel cylinders 48, 50. On trucks, one of the master cylinder outputs is often connected to the two front wheel cylinders, and the other master cylinder output is connected to the two rear wheel cylinders. On automobiles, each of the master cylinder outputs is normally connected to one of the front wheel cylinders and one of the rear wheel cylinders.
For proper braking operation, the master cylinder 24 must displace a volume of hydraulic fluid from each of its outputs 40, 42 that is sufficient to properly operate two of the wheel cylinders. Further, that hydraulic fluid displacement and the resulting braking force must be applied so that the vehicle is stopped in a desired distance. In one example, over a typical range of vehicle operation, it is necessary that a range of braking forces, for example, about 2000-8000 pounds, be provided by the wheel cylinders 44-50. In the absence of the vacuum booster 26, the mechanical and hydraulic systems within the braking system 18 cause the wheel cylinders 44-50 provide a braking force of about 1000 psi. Thus, the vacuum booster 26 is effective to provide an additional braking system pressure, so that the wheel cylinders 44-50 are able to supply the necessary braking forces.
While the engine is running, the vacuum system 30 generates a sufficient vacuum to provide the required power assisted braking. However, as will be appreciated, if for some reason the vacuum system 30 or vacuum booster 26 unexpectedly fail or an operator attempts to apply the brakes without the engine running, the braking system 18 loses a substantial portion of the available braking force. In the example above, when the vacuum power assist is lost, the braking system loses about 75% of its braking force. In that situation, with a limited braking force, the operator has a very difficult time bringing the vehicle to a stop within a reasonable stopping distance. Thus, there is a continuing effort to improve the braking capability of the braking system 18 in a manual-braking mode, that is, a braking mode having no power assist from the engine.
Consequently, there is a need for an improved braking system that can operate in a manual or nonpowered mode and be able to provide a substantially greater braking force than known systems.
The present invention provides a vehicle braking system having an improved vehicle braking capability when a braking power assist or boost is inoperative, which may occur either when the vehicle engine is off, or there is a failure in the power boost system, or the vehicle engine is not operating normally. Under such conditions, the vehicle braking system of the present invention provides a substantially greater braking force in response to a force applied by an operator on a brake pedal. Thus, the vehicle braking system of the present invention substantially improves the operator""s capability of stopping the vehicle in a shorter time and over a shorter distance under nonpower assist conditions.
According to the principles of the present invention and in accordance with the preferred embodiments, the invention provides an apparatus for use with a vehicle braking system having a brake pedal adapted to be used by an operator. A pushrod has one end connected to the brake pedal and an opposite end connected to a master cylinder. A hydraulic pressure intensifier has an intensifier inlet fluidly connected to the master cylinder and an intensifier outlet fluidly connected to a wheel cylinder. A bypass valve has an input and output fluidly connected to the intensifier inlet and the intensifier outlet, respectively. The bypass valve has an open state that fluidly connects the master cylinder to the wheel cylinder via the bypass valve, and a closed state that fluidly connects the hydraulic pressure intensifier between the master cylinder and the wheel cylinder. Thus, a fluid pressure from the master cylinder that is applied to the inlet of the intensifier cylinder is multiplied to a greater fluid pressure at the outlet of the intensifier cylinder and applied to the wheel cylinder.
In one aspect of the invention, the hydraulic pressure intensifier includes an intensifier cylinder with a larger cross-sectional area adjacent the intensifier inlet and a smaller cross-sectional area adjacent the intensifier outlet, and an intensifier piston having a larger cross-sectional area disposed in the larger cross-sectional area of the intensifier cylinder and a smaller cross-sectional area disposed in the smaller cross-sectional area of the intensifier cylinder.
In another aspect of the invention, hydraulic pressure intensifier includes a bypass valve actuator that may be implemented with a pressure sensor fluidly connected to a vacuum system. The pressure sensor provides an output signal having a first signal state in response to a lower pressure in the vacuum system for opening the bypass valve and a second signal state in response to a higher pressure in the vacuum system for closing the bypass valve.
In another embodiment of the invention, a method is provided for operating a vehicle braking system with a power assist with the above described pressure intensifier.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.